1. Field of the Invention
This invention relates to automatic fine tuning (AFT) circuits for controlling the frequency of television receiver tuners. More particularly, the invention is directed to a controllable AFT circuit that operates over a relatively wide frequency range covering a greater frequency band than the band of a single television channel.
2. Description of the Prior Art
AFT circuits for controlling the frequency to which television tuners are tuned have been described heretofore in companion cases U.S. Patent Application Ser. No. 716,655, filed Aug. 23, 1976, now U.S. Pat. No. 4,085,371 and U.S. Patent Application Ser. No. 752,427, filed Dec. 20, 1976, and both assigned to the assignee of the present application. Such AFT circuits translate the difference between the frequency f.sub.o of a desired television channel and the actual tuned frequency into a digital signal which is then used, in conjunction with a stored digital code, to derive an analog signal to be applied to a tuning element, such as a variable capacitance diode, that responds to voltage amplitude to control its reactance. Such a diode can be used in a circuit tunable over the television frequency bands, both the uhf and the vhf bands.
The automatic fine tuning circuit is desirable because various factors affect the tuning of the circuit. For example, temperature variations, aging of the components and other environmental factors can cause the tuning to shift from the desired frequency. Thus, even when a digital code corresponding to the analog voltage required to be applied to the voltage responsive reactance is recorded in a memory device it is not certain that the receiver will always return to exactly the proper frequency when that code is activated. Automatic adjustment of the code and therefore of the tuned frequency, is necessary to achieve the desirable result of unattended operation of the tuner.
One of the limitations of the AFT circuits in the companion applications is that they are arranged to operate in a relatively narrow band width of about .+-.1.2mHz on either side of the desired carrier frequency f.sub.o. If the frequency of the local osillator shifts outside of this band of f.sub.o .+-.1.2mHz, the AFT circuits in the companion applications will not work. One possibility for shifting the frequency too far is that the sound carrier of an adjacent channel, which may be only 1.5mHz away from the desired video carrier, may pull the AFT circuit into tuned relationship with that undesired sound carrier. Another possibility for permitting the tuning to drift outside of the AFT range arises if the receiver is allowed to continue to operate after the transmitter has gone off the air. Under such circumstance the AFT circuit may attempt to tune to a signal that is not present, and if that happens, the frequency to which the tuner is adjusted may be so far out of the desired range that when the channel does come back onto the air the AFT circuit will be unable to bring the tuner back to the proper frequency.